1. Field of the Invention
This invention relates to an assembly for use in a selective transfer process which treats a sheet of material by passing it between heated rollers. Specifically, the invention relates to improved assemblies for selective color transfer to sheets carrying xerographic images.
2. Background Art
The method for obtaining a copy of an original by electrostatic imaging is well known in the art, and is described in U.S. Pat. Nos. 2,397,691 and 2,357,809 to Carlson. The method generally comprises:
(1) electrostatically charging a photosensitive plate;
(2) irradiating the plate with actinic radiation to dissipate the charge in exposed areas and form an electrostatic image in unexposed areas;
(3) dusting the plate with a pigmented resin powder carrying an opposite electrostatic charge to the charge of the electrostatic image, such that the powder is attracted and then attached to the image area;
(4) transferring the image from the plate to a copy sheet by mechanical or electrical means; and
(5) fixing the powdered image on the copy sheet by heat or chemical treatment.
Various patents have disclosed methods for obtaining color copies. For example, U.S. Pat. No. 3,057,720 to Hayford et al. discloses a method for xerographic color reproduction, wherein the same general xerographic steps as outlined above are used in combination with the known "subtractive principle" of mixing primary colors, in a manner fully described in the '720 patent. U.S. Pat. No. 3,088,402 to Newman discloses a method of producing an imaged hectograph master by superposing a transfer sheet coated with a transferable layer of dye-carrying composition upon a sheet including a xerographic image and heating the sheets to render the xerographic image tacky. The transfer sheet is then stripped from the resulting imaged hectograph master. Similarly, U.S. Pat. No. 4,006,267 to Kurz, deceased, discloses placing a colored transfer donor in face-to-face contact with an image portion of a xerographic copy, heating the xerographic image, and removing the transfer donor to transfer the color of the donor layer to the image portion of the xerographic copy.
Methods for the selective transfer of metallic foils have also been disclosed. For example, U.S. Pat. No. 4,053,344 to Hirahara discloses placing a stamping foil, having an adhesive on one side, over ink which is dried until tacky. The adhesive is of a type which reacts with and adheres to the ink but does not adhere to the article. U.S. Pat. No. 3,519,512 to Downs similarly discloses using an ink-type "sensitizer" to adhere metallic foil to a substrate.
There are several difficulties encountered while developing a transfer sheet that consistently transfers a well defined overlay to a xerographic toner image substrate. This is particularly true When providing a selective transfer to a toner image produced by common office photocopy equipment.
One difficulty encountered is that many toners include a release component to aid in removal of the toner from the plate once the toner is bound to the copy sheet. The copy sheet is generally a porous paper, to which the toner powders mechanically adhere as they are fused. Thus the toner can still bond to the copy sheet while the release component prevents adherence of the toner to photocopy machine elements.
Beside release components included in the makeup of the toner itself, silicone oil is often applied to machine parts to prevent the toner from sticking to them. During operation of the photocopy machine, this silicone oil coats the exposed surface of the xerographic image. Thus, when attempting to bond a transfer layer to the image, the silicone oil acts to prevent such bonding.
Consistent coverage of the xerographic image is difficult to achieve because of differences in the toner compositions of different manufacturers, differences in photocopy machine operation, etc. If the transfer layer adheres too readily, bridging occurs. Bridging is a term used to describe the situation where a single continuous portion of the transfer layer adheres to two or more discontinuous portions of the xerographic toner image, covering a portion of the copy sheet which does not have toner on it. A simple illustration of bridging is the filling in of images of letters or numbers normally having a void space, such as "a", "e", "o", etc.
On the other hand, the makeup of a toner xerographic image of one particular letter or other element of the total image is seldom uniform across the surface area of the letter or element. Thus, if the transfer layer does not adhere well enough to the toner, only parts of the image will be selectively coated.
Another problem which is encountered with selective transfer to a xerographic image on sheet material, such as a piece of paper, is keeping the transfer sheet clean and unwrinkled until it is used, and then properly aligning the transfer material and the sheet material during use. Since the transfer material described herein is particularly well suited for making transfers to common photocopies, there is a need for an easy to use method of making the transfer so that it can be done by individuals with minor training, in diverse locations and on a sporadic basis, so that the transfer process itself can be utilized to its full capacity.
Where the transfer process is performed using heated rollers, presumably encased in a piece of machinery, it is important that sheet material fed into the machine not stick to the rollers, or otherwise become lodged in the machine.